In recent years, IC cards have been used for tickets in traffic facilities such as trains, buses, etc., electronic money, etc. Such IC cards exchange information with reader writers via near field communication (so-called contactless communication). This communication is standardized in accordance with, for example, the international standard ISO/IEC 14443-2 (Type A and Type B) or the international standard ISO/IEC 18092. In these standards, reader writers (initiators) and IC cards (targets) are specified to perform communication by means of ASK (Amplitude Shift Keying) modulation.
Various techniques have been disclosed concerning demodulation circuits used for such near field communication. For example, Patent Literature 1 discloses a demodulation circuit that performs demodulation with orthogonal detection (in a phase change demodulation mode) in an application to a reader writer, and performs demodulation with analog envelope detection or the orthogonal detection (in an amplitude change demodulation mode) in an application to an IC card. Also, Patent Literature 2 discloses a demodulation circuit for a reader writer; the demodulation circuit includes an ASK detection circuit and a phase detection circuit, and uses, out of an output signal of the ASK detection circuit and an output signal of the phase detection circuit, either one with a higher average amplitude level.